1. Field of the Invention
This invention relates to polyvinyl alcohol and more specifically it relates to stable aqueous polyvinyl alcohol/melamine-formaldehyde resin interaction products.
2. Description of the Prior Art
There are numerous commercial wet end additives being used in paper making at the present. These have many limitations. Some examples of the additives used most commonly, are as follows.
Cationic starches are often used for improving retention of cellulosic fines, filler and pigment, and also for increasing the dry strength of the resulting paper. However, such improvements are generally modest, and at the same time cationic starch use can lead to irregularities in performance (irreproducibility of batches, low solution stability, low wet strength), incompatibility with other components in the furnish (alum, size, other salts), and high biological oxygen demand (BOD) for additive not on the pulp or recycled, and which is lost in the waste water.
Other wet-end additives are often used to confer permanent wet strength to the resulting paper, such as cationic urea-formaldehyde UF resins, amine-containing polyamides treated with epoxides (e.g., Hercules' "Kymene"557) or melamine-formaldehyde (MF) resins (e.g., "Parez"607 of American Cyanamid). However, UF resins are slow curing on the machine, while the polyamides are relatively expensive, slow to adsorb on the cellulose pulp, and make repulping of the paper relatively difficult. The MF resins show poor pigment and filler retention, and also exhibit low water absorbency, whereas absorbency is often desired along with wet strength. All of these types of additives give only modest enhancement of dry strength. Also, none of the above types are now recognized as improvers of wet web strength (at their usual concentration of application) which would permit greater production control and in some cases, increased productivity.
Conventional soluble polyvinyl alcohol in the form of powder, granules or chopped fiber has been used in Japan as an additive in the wet-end of a paper-making machine. Increased paper strength and oil resistance was disclosed. However, careful control of polyvinyl alcohol particle size, preliminary heat treatment, and degree of hydrolysis of the polyvinyl alcohol is required. Similarly, careful control of the temperature, and level of water pickup by the particles and the forming paper is required before passing through the drier rolls (see "Polyvinyl Alcohol", edited by C. A. Finch, Wiley, N.Y. (1973) pp 301-305). As these particles are nonionic, low retention of fines would be expected and as they are non-curing have no wet strength capability.
The use of melamine-formaldehyde resins as wet-end additives to give high wet strength papers is well known (see C. S. Maxwell's review in TAPPI Monograph No. 29, "Wet Strength in Paper and Paperboard, Editor J. P. Weidner (1965), pages 20-32.
The interaction of starch and the cationic precondensate of melamine-formaldehyde to make a cationic starch (promoting adsorption of the binder) is disclosed in U.S. Pat. No. 2,998,344 (cf. column 3). No concentration effects were indicated as important. Also the product was unsatisfactory per se as a binder for cellulosic pulp (cf. column 6).
U.S. Pat. No. 3,594,271 disclosed aqueous acidic colloidal solutions of a cationic reaction product of a cationic thermosetting melamine-formaldehyde acid colloid with 5 to 50 times its weight of a water soluble starch and the process of treating paper therewith. Such products are disclosed to give good adsorption on the fibers and enhanced dry strength coupled with low wet strength. Total solids content of the mixture is 2-10%, but it is indicated that "this is not critical," (column 2). It was also disclosed that the low wet strength is a result of the low concentration of the melamine-formaldehyde resin relative to the starch.
U.S. Pat. No. 3,424,650 disclosed that starch reacted with formaldehyde-guanidine-melamine resins was rendered much more adsorptive to cellulose fibers. Inclusion of all three materials as reactants in 9-14/0.4-1.6/0.4-1.6 ratios, respectively, was critical in order to produce a relatively stable resin and for sufficient activity for increasing dry strength of paper articles by prior reaction with starch. Also concentration of reactants can be 1-40% by weight.
A combination of a guanidine-formaldehyde resin and a hydrocolloid (such as starch or polyvinyl alcohol), in about 2/1 ratio by weight, is disclosed in U.S. Pat. No. 3,002,881 as a good wet end additive, increasing the wet strength of the resulting paper. Presumably, components are added independently to the dilute pulp slurry. There is no indication of prereaction of resin and polyvinyl alcohol, because of lack of stability of premixes (cf. column 5).
The advantages of using a cationic material (cationic starch) at the wet-end of a papermaking machine are disclosed in U.S. Pat. No. 4,029,885.
Other methods to prepare cationic highly adsorbant polyvinyl alcohols have also been described. However, these would be at higher costs and/or would offer problems with a toxic reactant. These are as follow:
U.S. Pat. Nos. 3,597,313 and 3,772,407 disclose copolymers of vinyl alcohol modified with cationic monomers.
U.S. Pat. No. 3,051,691 discloses that polyvinyl alcohol and calcium cyanamide form cationic polymeric polyols that are substantive to cellulose.
The use of polyvinyl alcohol plus methylol melamines (the monomeric -melamine-formaldehyde) in paper coatings is described in British Pat. No. 551,950 Application as a textile finish is disclosed in U.S. Pat. No. 2,876,136. In the latter, reaction between the two components probably didn't take place until after application to the substrate (the catalyst was added at this point). The polyvinyl alcohol/methylol compound ratio disclosed was from 1/1 to 1/125.
U.S. Pat. No. 3,067,160 disclosed that the addition of even small amounts of polyvinyl alcohol to cationic melamine-formaldehyde resin acid colloids (methyl ether form) was unsuccessful. Such systems were very unstable and gelled thus indicating that one would not expect stable melamine-formaldehyde acid colloid solutions containing polyvinyl alcohol.
It has been known for a long time that addition of a crosslinking agent to polyvinyl alcohol in aqueous solution at moderate-to-elevated concentrations, will lead to gel formation, but if the solution is dilute enough intramolecular interaction will occur almost exclusively, so that no gel would form (cf. W. Kuhn and G. Balmer, Journal of Polymer Science Vol. 57 page 311-319 (1962). Further the work of these authors indicates that in reaction of a polyvinyl alcohol having a degree of polymerization on the order of 1000-2000, with a highly functional cross-linking agent such as the melamine-formaldehyde resin acid colloid (and with the latter at a concentration of as high as 20 to 100% by weight of the PVA), a very low concentration of the PVA must be present (est. 0.3-0.5%) to prevent gelation. Thus it is surprising that interaction of PVA and the MF resin acid colloid can occur in solution concentrations as high as 3.75% to give stable yet active systems even with extensive heating of the solution.
These authors indicate that complete separation of the polymer chains is necessary for this dilution effect to occur. Other workers also indicated that complete separations of molecules of polyvinyl alcohol require concentrations below 0.25%., and that if concentration is increased to about 0.9% the swollen polymer coils must interpenetrate, and entanglements become quite important [cf. "Polyvinyl alcohol", by J. G. Pritchard, Gorden and Breach, NYD, (1979), page 15].
U.S. Pat. No. 3,630,831 disclosed a suspension of a binding agent for nonwoven materials prepared by dispersing particles of a polymer swellable in cold water and soluble in warm water (e.g., starch or polyvinyl alcohol) in a solution of a cross-linking agent (e.g., formaldehyde) and a cross-linking catalyst (e.g., HCl) wherein the particles swell and absorb the solution. At room temperature, the cross-linking agent partially reacts and the suspension is then diluted. The binding agent is added to a fiber slurry and a web is formed which is heated to at least 140.degree. C. to complete the cross-linking of the polymer.